1. Technical Field of the Disclosure
The present invention relates to a mechanism for feeding and transporting printing plates to an exposure table, with the aid of rollers.
2. Description of the Prior Art
Lettered and imaged printing masters which are directly suitable for printing are manufactured in such a way that the printing plates are first electrostatically charged and that an original is then projected imagewise upon the individual plate. After this exposing procedure, the printing master is developed with a developer, fixed and decoated and is then ready for printing.
U.S. Pat. No. 4,006,984 discloses an apparatus for the manufacture of printing masters by electrophotographic means, in which the individual printing plate is picked up from a stack in a plate holder by a transport device comprised of a transport carriage with a reduced pressure-suction means. The transport carriage transfers the printing plate to an exposure platform. Charging of the individual printing plate and transporting it to the exposure station are combined in a time-saving manner by mounting at the front of the transport carriage a corona charging station for the electrostatic charging of the printing plates. The corona charging station extends transversely to the direction of motion of the carriage. The carriage runs on two guide rails and is driven by a motor positioned on top of it. The carriage engages via a gear wheel with a rack-and-pinion gear which is arranged parallel to the direction of motion of the transport carriage. The transport carriage has a vacuum plate which is attached to its underside and which is, by a number of holes, connected with a vacuum pump. When the transport carriage is placed upon the upper side of the printing plate which lies on top in the plate holder, reduced pressure is generated, and the printing plate is sucked to the vacuum plate. The transport carriage is then displaced in the direction of the exposure platform by means of the motor. As soon as the carriage has reached its position above the exposure platform, it is lowered. The vacuum is released so that the printing plate is set free and is deposited on the exposure platform. The exposure platform is also designed as a vacuum plate; and, under the action of reduced pressure, the printing plate is firmly urged against the platform.
In this previously proposed apparatus, the photoconductive, coated side of the printing plate is touched by suction means when the individual printing plate is picked up from the stack. Compared with rollers or webs which, in other known apparatuses, are guided over the coated side of the printing plate, the suction means allow a considerably gentler transport of the printing plate. Nevertheless, even with the application of suction means, mechanical or physical defects on the developed printing master cannot be precluded with certainty.
In U.S. Pat. No. 4,149,798, a transport station for printing plates is described, which includes a stacking area containing a supply of printing masters. A paper disposal area is provided for the sheets of paper which are interposed between the individual masters in the stack. By means of a control mechanism equipped with a number of suction cups, the printing masters are transported from the stacking area to a conveyor. The control mechanism is pivotally attached to a support arm which is connected to a motor via a gear mechanism. Thus, the control mechanism is adapted to perform a lateral sliding movement back and forth. The uppermost printing plate in the stack is picked up by at least four suction cups of the control mechanism. Then the control mechanism swings upwards; and, at the same time, the support arm mechanically approaches the conveyor. When the vacuum in the suction cups is released, the control mechanism pivots downwards and deposits the printing plate on the conveyor. A second control mechanism positioned above the paper separation sheet engages the sheet, picks it up and conveys it to the paper disposal area.
The conveyor transports the printing master to an exposure platform which has a plurality of holes in its upper surface and which comprises a chamber connected to a vacuum pump by a vacuum line. After the printing master has adopted the correct position on the surface of the exposure platform, a vacuum is generated; and the master is thus securely retained on the platform. The exposure system used to expose the printing master comprises a movable carriage to which a corona charging device and a light reflecting mirror are attached. A laser which emits modulated laser light is arranged in such a way that the light scan emitted by the laser is deflected by the mirror and impinges upon the printing master retained on the exposure platform in a plane which is approximately perpendicular to the surface of the master.
As in the above-described prior art apparatus, the printing plate lying on top in the stack is picked up in such a way that its photoconductive layer comes into contact with suction means; and this may lead to mechanical or physical defects on the finished printing master.